Regenerative refrigerator

ABSTRACT

A disclosed regenerative refrigerator including a regenerator filled with a regenerative material for accumulating cooling of a refrigerant gas, wherein the regenerator is divided into a central region and a peripheral region on a cross-sectional face of the regenerator, and a specific heat of the central region is larger than a specific heat of the peripheral region.

RELATED APPLICATIONS

Priority is claimed to Japanese Patent Application No. 2012-161531 filedon Jul. 20, 2012, the entire contents of which are incorporated hereinby reference.

BACKGROUND

1. Technical Field

The present invention generally relates to a regenerative refrigerator.

2. Description of the Related Art

In general, a Gifford-McMahon (GM) refrigerator, a pulse tuberefrigerator, or the like is known as a regenerative refrigerator forcooling an object by an adiabatic expansion of a refrigerant gas andaccumulating cooling generated by the adiabatic expansion of therefrigerant gas. These regenerative refrigerators include a regeneratorfor accumulating cooling generated when the refrigerant gas isadiabatically expanded. A regenerative material is filled in theregenerator in order to accumulate cooling as disclosed in JapaneseLaid-open Patent Publication No. 2008-224161. For example, lead is usedas the regenerative material.

SUMMARY

One aspect of the embodiments of the present invention may be to providea regenerative refrigerator including a regenerator filled with aregenerative material for accumulating cooling of a refrigerant gas,wherein the regenerator is divided into a central region and aperipheral region on a cross-sectional face of the regenerator, and aspecific heat of the central region is larger than a specific heat ofthe peripheral region.

Additional objects and advantages of the embodiments are set forth inpart in the description which follows, and in part will become obviousfrom the description, or may be learned by practice of the invention.The objects and advantages of the invention will be realized andattained by means of the elements and combinations particularly pointedout in the appended claims. It is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory and are not restrictive of the invention asclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the inside of a regenerativerefrigerator of a first embodiment;

FIG. 2 is a cross-sectional view taken along a line A-A of FIG. 1;

FIGS. 3A and 3B illustrate flow rate distribution of a refrigerant gasinside a second stage displacer;

FIG. 4 schematically illustrates the inside of a regenerativerefrigerator of a second embodiment;

FIG. 5 schematically illustrates the inside of a regenerativerefrigerator of a third embodiment;

FIG. 6 schematically illustrates the inside of a regenerativerefrigerator of a fourth embodiment; and

FIGS. 7A, 7B, and 7C are plan views for explaining a filler provided ina regenerative refrigerator of a fourth embodiment.

DETAILED DESCRIPTION

In a regenerative refrigerator realizing an ultralow temperature of 30Kor less, the specific heat of lead suddenly decreases along with adecrement of a temperature in a temperature range of 15K or less. Theregenerative effect may not be sufficient when lead is used as aregenerative material.

Then, it is possible to use a magnetic regenerative material such asHoCu₂ or the like having a specific heat larger than that of lead in thetemperature range of 30K or less. A magnetic regenerative material showsphase transition in a temperature range of 15K or less so as to changeto an anti-ferromagnetic material. Because the magnetic regenerativematerial has a magnetic susceptibility larger than lead or the like,high-efficiency regenerative effect is possible.

However, the magnetic regenerative material is mainly made of arare-earth material. Therefore, it is difficult to obtain the magneticregenerative material and the cost of the magnetic regenerative materialis high.

The embodiments are provided in consideration of the above problems. Oneobject of the embodiments is to provide a regenerative refrigeratorwhich can provide a high-efficiency regenerative effect at a low cost.

In the disclosed regenerative refrigerator, a specific heat in a centralregion where the flow rate of a refrigerant gas is high is increased tobe larger than the specific heat in a peripheral region where the flowrate of a refrigerant gas is low. Therefore, the regenerator efficiencycan be enhanced.

A description is given below, with reference to the FIG. 1 through FIG.7C of embodiments of the present invention. Where the same referencesymbols are attached to the same parts, repeated description of theparts is omitted.

First Embodiment

FIG. 1 is a cross-sectional view of a regenerative refrigerator of afirst embodiment. Within the first embodiment, a two-stageGifford-McMahon (GM) refrigerator using a helium gas as a refrigerantgas is exemplified as a regenerative refrigerator 1A. However, the firstembodiments are applied to not only the GM refrigerator but also variousrefrigerators (for example, a pulse tube refrigerator or the like) whichhave a regenerator filled with a regenerative material.

The regenerative refrigerator 1A includes a first stage displacer 2, asecond stage displacer 20, a first stage cylinder 4, a second stagecylinder 30, a first stage cooling stage 5, a second stage cooling stage27, a first stage regenerator 17, a second stage regenerator 26, acompressor 12, and so on.

The first stage displacer 2 has a cylindrical shape. The first stagedisplacer 2 includes a first stage displacer main body 2A, a first stageheat exchanging portion 2B, a first stage regenerator 17, and so on. Thefirst stage displacer main body 2A is shaped like a cylinder having abottom. A regenerative material 7 is filled in the first stage displacermain body 2A. The first stage regenerator 17, which is filled with theregenerative material 7, is provided. The regenerative material 7 may bemade of lead, copper, or the like having a large specific heat (avolumetric specific heat) in a temperature range of 15K or higher.

A flow smoother 9 is provided on the high temperature side of the firststage regenerator 17 in order to control a flow of a refrigerant gas. InFIG. 1, the upper side corresponds to the high temperature side. A flowsmoother 10 is provided on the low temperature side of the first stageregenerator 17 in order to control the flow of the refrigerant gas. InFIG. 1, the lower side corresponds to the lower temperature side.

On a high temperature end of the first stage displacer 2, a first flowpath 11 is formed to allow a refrigerant gas to flow from the roomtemperature chamber 8 to the first stage regenerator 17, which is formedon the high temperature side of the first stage displacer 2. The roomtemperature chamber 8 is a space formed between the upper surface of thefirst stage cylinder 4 and the upper surface of the first stagedisplacer 2. A supply and discharge system (described later) isconnected to the room temperature chamber 8.

On a low temperature end of the first stage displacer 2, a first stageheat exchanging portion 2B is provided. Between the first stagedisplacer main body 2A and the first stage heat exchanging portion 2B, asecond flow path 16 is formed to connect the first stage regenerator 17to a first stage expansion space 3. The first stage heat exchangingportion 2B is connected to the first stage displacer 2 using a pin 6.

The first stage expansion space 3 is a space formed between the lowersurface of the first stage cylinder 4 and the lower surface of the firststage heat exchanging portion 2B (first stage displacer 2). A highpressure refrigerant gas is introduced into the first stage expansionspace 3 via the second flow path 16. A first stage cooling stage 5 isprovided at a position corresponding to the first stage expansion space3 of the first stage cylinder 4.

The above first stage displacer 2 is installed in the first stagecylinder 4. A driving mechanism (not illustrated) such as a scotch yokemechanism is connected to the high temperature end of the first stagedisplacer 2. With the above scotch yoke mechanism, the first stagedisplacer 2 reciprocates in the first stage cylinder 4 by the scotchyoke mechanism.

A seal 15 is installed at a predetermined position between the firststage displacer 2 and a top flange. The seal 15 hermetically divides thefirst stage expansion space 3 from a room temperature chamber 8.

The second stage cylinder 30 is integrally formed on a low temperatureend portion of the first stage cylinder 4. The second stage cylinder 30accommodates the second stage displacer 20 so that the second stagedisplacer 20 is movable in the second stage cylinder 30.

The second stage displacer 20 is in a cylindrical shape and is connectedto the low temperature end portion of the first stage displacer 2.Specifically, a pin 19 a is installed in the low temperature end of thefirst stage heat exchanging portion 2B. A pin 19 b is installed in thehigh temperature end of the second stage displacer 20. The pins 19 a and19 b are connected by a connector 19 c. Thus, the second stage displacer20 is connected to the first stage displacer 2.

Therefore, while the first stage displacer 2 reciprocates inside thefirst stage cylinder 4 by the scotch yoke mechanism, the second stagedisplacer 20 also reciprocates in the second stage cylinder 30 alongwith the reciprocation of the first stage displacer 2.

The second stage displacer 20 includes a second stage displacer mainbody 20A, a second stage heat exchanging portion 20B, a second stageregenerator 26, and so on. A second stage displacer main body 20A is ina cylindrical shape having a bottom, and has a second stage regenerator26 in the second stage displacer main body 20A. The above second stagedisplacer 2 is installed in the second stage cylinder 30.

On the high temperature end of the second stage displacer 20, a thirdflow path 24 is formed to allow the refrigerant gas to flow from a firststage expansion space 3 to the second stage regenerator 26 formed on thehigh temperature side of the second stage displacer 20. On a lowtemperature end of the second stage displacer 2, the second stage heatexchanging portion 20B is installed. Between the second stage displacermain body 20A and the second stage heat exchanging portion 20B, a fourthflow path 29 is formed to connect the second stage regenerator 26 to asecond stage expansion space 28.

The second stage expansion space 28 is a space formed between the lowersurface of the second stage cylinder 30 and the lower surface of thesecond stage heat exchanging portion 20B (second stage displacer 20). Ahigh pressure refrigerant gas is introduced into the second stageexpansion space 28 via the fourth flow path 29. A second stage coolingstage 27 is provided at a position corresponding to the second stageexpansion space 28 of the second stage cylinder 30.

The supply and discharge system includes a compressor 12, a supply valve13, a return valve 14, and so on. When the supply valve 13 is opened andsimultaneously the return valve 14 is closed, a high pressurerefrigerant gas, which is generated by the compressor 12, is suppliedinto a room temperature chamber 8. In an opposite manner, when thesupply valve 13 is closed and simultaneously the return valve 14 isopened, a low pressure refrigerant gas flows back into the compressor12.

Next, operations of the above described regenerative refrigerator 1A aredescribed.

When the supply valve 13 is opened while the first and second stagedisplacers 2 and 20 are at the lower dead ends, the refrigerant gas fromthe compressor 12 flows into the first stage regenerator 17 via the roomtemperature chamber 8 and the first flow path 11. The high pressurerefrigerant gas, which is cooled by exchanging heat with theregenerative material 7 in the first stage regenerator 17, is suppliedinto the first stage expansion space 3 via the second flow path 16.

The refrigerant gas supplied to the first stage expansion space 3 flowsinto the second stage regenerator 26 via the third flow path 24. Therefrigerant gas exchanges heat with regenerative materials 40 and 42(described below) so as to be cooled and is supplied to the second stageexpansion space 28 via the fourth flow path 29.

Under the condition, the first and second stage displacers 2, 20 aremoved toward the upper dead end by the scotch yoke mechanism. With this,the volumes of the first and second stage expansion spaces 3 and 28 areincreased. At this time, the refrigerant gas continues to be supplied tothe first and second stage expansion spaces 3 and 28 via the first andsecond regenerators 17 and 26.

When the first and second stage displacers 2 and 20 move in the vicinityof the upper dead end, the supply valve 13 is closed and the returnvalve 14 is opened. With this, the refrigerant gas expands in the firstand second stage expansion spaces 3 and 28 thereby generating cooling.

The expanded refrigerant gas flows back to a low pressure side of thecompressor 12 via the first and second stage regenerators 17 and 26 andthe flow paths 11, 16, 24, and 29. At this time, the regenerativematerials 7, 40, and 42 in the first and second regenerators 17 and 26accumulate cooling of the refrigerant gas.

While the return valve 14 is maintained to be opened and the supplyvalve 13 is maintained to be closed, the first and second stagedisplacers 2 and 20 move toward the lower dead end.

By repeating a cycle of the above operations, the first stage expansionspace 3 is cooled to be, for example, about 40K and the second stageexpansion space is cooled to be, for example, about 4K.

Referring to FIGS. 1 to 3B, the second stage regenerator 26 provided inthe second stage displacer 20 is described in detail.

FIG. 2 is a cross-sectional view of the second stage displacer 20illustrated in FIG. 1 taken along a line A-A. FIG. 3 illustrates a flowdistribution of the refrigerant gas flowing through the second stagedisplacer 20.

Referring to FIG. 1, the second stage regenerator 26 has a separatingmember 31 on the high temperature side and a separating member 32 on thelow temperature side. The regenerative materials 40 and 42 fill a spaceformed by the separating members 31 and 32. Although the separatingmembers 31 and 32 prevent the regenerative materials 40 and 42 fromflowing therethrough, the refrigerant gas can freely pass through theseparating members 31 and 32.

Within the first embodiment, a cross-sectional face of the second stageregenerator 26 is divided into a central region 21, which is shapedsubstantially like a circle and positioned in the vicinity of thecenter, and a peripheral region 22, which is shaped like a ring andpositioned around the central region 21.

Here, a flow rate of the refrigerant gas in the second stage regenerator26 is described. As described above, when the regenerative refrigerator1A cools an object, the refrigerant gas flows through the inside of thesecond stage displacer 20. While the supply valve 13 is opened, therefrigerant gas flows through the high temperature end to the lowtemperature end inside the second stage displacer 20 (in the downwarddirection in FIGS. 1 and 3B). While the return valve 14 is opened, therefrigerant gas flows through the low temperature end to the hightemperature end inside the second stage displacer 20 (in the upwarddirection in FIGS. 1 and 3A).

FIG. 3A illustrates a flow distribution of the refrigerant gas insidethe second stage displacer 20 from the low temperature end to the hightemperature end. FIG. 3B illustrates a flow distribution of therefrigerant gas inside the second stage displacer 20 from the hightemperature end to the low temperature end. The lengths of arrows inFIGS. 3A and 3B correspond to the flow rates of the refrigerant gasflowing in the second stage regenerator 26.

Referring to FIGS. 3A and 3B, the flow distribution of the refrigerantgas flowing through the second stage displacer 20 is not even in aflowing direction of the refrigerant gas.

In other words, on the cross-sectional face of the second stagedisplacer, the flow rate (hereinafter, a “central region flow rate”) ofthe refrigerant gas is larger in the central region 21 of second stagedisplacer 20. Meanwhile, the flow rate (hereinafter, a “peripheralregion flow rate”) of the refrigerant gas on the peripheral region 22 isless than that of the central region 21 of the second stage displacer20. This is because the flow path resistance of the refrigerant gas onthe central region 21 is less than the flow path resistance of therefrigerant gas on the peripheral region 22.

Within the first embodiment, in association with the flow distributionof the refrigerant gas inside the second stage regenerator 26, thesecond stage displacer 20 is divided into the central region 21 and theperipheral region 22 on the cross-sectional face. Specifically, bydividing the separating member 33 (corresponding to a separating memberrecited in claims) in the above cylindrical shape, which is provided ina boundary between the central region 21 and the peripheral region 22,to thereby divide the central region 21 and the peripheral region 22.

The separating member 33 is provided in an upper portion of theseparating member 32, which is provided on the low temperature end sideinside the second stage regenerator 26. The separating member 33 allowsthe refrigerant gas to pass through in a manner similar to otherseparating members 31 and 32. However, the separating member 33 preventsthe regenerative material from passing through.

On the other hand, within the first embodiment, two types of thenonmagnetic regenerative material 40 and the magnetic regenerativematerial 42 are used as the regenerative material filling the secondstage regenerator 26. Within the first embodiment, bismuth or an alloycontaining bismuth is used as the nonmagnetic regenerative material 40.HoCu₂ is used as the magnetic regenerative material 42.

The magnetic regenerative material 42 such as HoCu₂ has a specific heat(a volumetric specific heat) larger than the nonmagnetic regenerativematerial 40 such as bismuth under an ultralow temperature of 30K orless. The second stage displacer 20 has an ultralow temperature of 15Kor less when the regenerative refrigerator 1A operates. Therefore, whenthe regenerative refrigerator 1A operates, the second stage regenerator26 has a temperature of 30K or less. The magnetic regenerative material42 has specific heat larger than the specific heat of the nonmagneticregenerative material 40.

Within the first embodiment, the magnetic material 42 having a largerspecific heat is provided in the central region 21. The magneticmaterial 40 having a less specific heat than that of the magneticregenerative material 42 is provided in the peripheral region 22.Therefore, the specific heat of the central region 21 becomes largerthan the specific heat of the peripheral region 22.

As described, within the first embodiment, because the magneticregenerative material 42 having a large specific heat is provided in thecentral region where the flow rate of the refrigerant gas is large, itis possible to enhance an efficiency of accumulating cooling of thesecond stage regenerator 26.

Because the magnetic regenerative material 42 is provided only in thecentral region 21, the filling amount (the amount to use) of themagnetic regenerative material 42 can be reduced in comparison with thestructure in which the magnetic regenerative material 42 is provided inthe entire second stage regenerator.

Thus, a sufficient cold accumulating capability can be obtained withless magnetic regenerative material, which is rare and expensive.

Further, in the first embodiment, the magnetic regenerative material 42is provided in the central region 21 in the vicinity of the lowtemperature end. In a case where HoCu₂ is used as the magneticregenerative material 42, the peak of the volume specific heat is as lowas 5K to 10K. Therefore, an efficiency of accumulating cooling is highby providing HoCu₂ on the low temperature end in the central region 21.

Within the first embodiment, the height of the separating member 33separating the nonmagnetic regenerative material 40 from the magneticregenerative material 42 is set to be less than the overall height ofthe second stage regenerator 26. The magnetic regenerative material 42is provided only in the vicinity of the low temperature end. Theseparating member 34 is provided in the upper portion of the magneticregenerative material 42, which fills the inside of the separatingmember 33, so that the nonmagnetic regenerative material 40 is not mixedwith the magnetic regenerative material 42. With this structure, theamount of the magnetic regenerative material 42 to be used can bereduced while maintaining heat exchanging efficiency with therefrigerant gas.

Within the first embodiment, bismuth is used as the nonmagneticregenerative material 40, and HoCu₂ or the like is used as the magneticregenerative material 42. However, the materials of the nonmagneticregenerative material 40 and the magnetic regenerative material 42 arenot limited to these. Other materials may be used. At this time, themagnetic regenerative material 42 is preferably made of a materialhaving a peak of the specific heat at 30K or less. Further, thenonmagnetic regenerative material 40 is preferably made of lead insteadof bismuth or the like. However, in consideration of the environment, itis preferable to use bismuth or the like.

For example, a ratio between cross-sectional areas of the central andperipheral regions is appropriately selected depending on the capabilityand the size of the refrigerator. It is preferable that the centralregion occupies from about 50% to about 95%.

Further, when the regenerative materials 40 and 42 fill the inside ofthe second stage regenerator 26, it is preferable to fill theregenerative materials 40 and 42 so that the pressure loss of therefrigerant gas flowing through the central region becomes greater thanthe pressure loss of the refrigerant gas flowing through the peripheralregion.

Next, referring to FIGS. 4 to 7C, regenerative refrigerators 1B to 1D ofsecond to fourth embodiments are described. Referring to FIGS. 4 to 7C,the same reference symbols are attached to the structures correspondingto the structures illustrated in FIGS. 1 to 3B and description of theseportions is omitted.

Second Embodiment

FIG. 4 schematically illustrates a regenerative refrigerator 1B of thesecond embodiment. Within the first embodiment, only one type of themagnetic regenerative material 42 is arranged in the central region 21in the regenerative refrigerator 1A of the above first embodiment.Within the second embodiment, two types of regenerative materials 50 aand 50 b are used as the regenerative material 50 having a peak of thespecific heat at 30K or less. The regenerative materials 50 a and 50 bare laminated via a separating plate 35.

Specifically, HoCu₂ being the magnetic regenerative material used in thefirst embodiment is used as the first regenerative material 50 a, whichis positioned on the upper side. Meanwhile, GOS (Cd₂O₂S) being aceramics regenerative material is used as the second regenerativematerial 50 b, which is positioned on the lower side. GOS has a specificheat of about two times of that of HoCu₂ in an ultralow temperatureregion of 4K to 5K. Therefore, the first and second regenerativematerials 50 a and 50 b are arranged in regenerative material 50 b madeof GOS is provided on the low temperature side of the position ofproviding the first magnetic regenerative material 50 a. Then, it ispossible to obtain a higher efficiency of accumulating cooling in thesecond embodiment than in the first embodiment.

Within the above second embodiment, GOS is used as the secondregenerative material 50 b, it is possible to use another regenerativematerial having a high specific heat peak in the ultralow temperaturesuch as GAP (GdAlO₃) instead of GOS.

FIG. 5 schematically illustrates a regenerative refrigerator 1C of thethird embodiment.

Within the above first embodiment, a two-stage regenerative refrigerator1A including two sets of the displacer, the cylinder, the regeneratorand so on is illustrated. However, this patent application is notlimited to the two-stage regenerative refrigerator.

Within the third embodiment, the magnetic regenerative material 62 isprovided in the central region 21 of a single-stage regenerativerefrigerator. A nonmagnetic regenerative material 64 is provided in theperipheral region 22 around the central region 21. In the single-stageregenerative refrigerator 1C, the regenerative materials 62 and 64 oftwo different types are used. The regenerative material 62 having a highspecific heat is filled in the central region 21, and the regenerativematerial 64 having a low specific heat is filled in the peripheralregion 22 to thereby perform an effect similar to the first embodiment.

The temperature inside the single-stage regenerative refrigerator 10 ishigher than the temperature inside a multi-stage regenerativerefrigerator. Therefore, in the single-stage regenerative refrigerator10, the regenerative material provided in the central region 21 is notlimited to a magnetic regenerative material and may be a material havinga lower specific heat than that of the magnetic regenerative material.Further, the nonmagnetic regenerative material other than the magneticregenerative material may be filled in the central region 21.

For example, a ratio between cross-sectional areas of the central andperipheral regions is appropriately selected depending on the capabilityand the size of the refrigerator. It is preferable that the centralregion occupies from about 50% to about 95%.

Fourth Embodiment

FIG. 6 schematically illustrates a regenerative refrigerator of thefourth embodiment.

The regenerative refrigerator 1D is separated into the high and lowtemperature sides by providing a separating member 36 inside the secondstage regenerator 26. The nonmagnetic regenerative material 40 fills theregion on the high temperature side (hereinafter, a “high temperatureregion” 26 a), and the magnetic regenerative material 42 fills theregion on the low temperature side (hereinafter, a “low temperatureregion” 26 b). Therefore, in the low temperature region 26 b of thesecond regenerator 26, the magnetic regenerative material 42 is providedin both of the central region 21 and the peripheral region 22.

Further, in the fourth embodiment, a filler 44A is provided in theperipheral region 22 of the magnetic regenerative material 42 on the lowtemperature side. FIG. 7A is an enlarged view of the filler 44A.

The filler 44A is formed of a plate material made of copper, a copperalloy or the like having high heat conductivity. The filler 44A is in aring shape (an annular shape) with a central hole 45 formed in thecenter. The diameter of the central hole 45 is substantially the same asthe diameter of the central region 21. The outer diameter of the filler44A is determined so that the filler 44A can be installed inside thesecond stage regenerator 26.

Further, plural through holes 46 are opened in the filler 44A. Withinthe fourth embodiment, 8 pairs of (two) through holes of the throughholes 46 are opened in a radial pattern. The diameters of the throughholes 46 are set to be larger than a particle diameter of the magneticregenerative material 42.

The above filler 44A is provided inside the second stage regenerator 26.At this time, the filler 44A is provided inside the second stageregenerator so as to be embedded in the regenerative material 42. Withinthe fourth embodiment, three sheets of the fillers 44A are piled with apredetermined gap inside the magnetic regenerative material 42. However,the number of the fillers 44A filling the inside of the magneticregenerative material 42 is not limited to the above and can beappropriately selected.

As described, the central hole 45 is opened in the filler 44A. Byproviding the filler 44A inside the magnetic regenerative material 42,the filler 44A is provided substantially in the peripheral region 22.

Here, a filling rate of the magnetic regenerative material inside thelow temperature region 26 b is described. In the low temperature region26 b, the filler 44A is provided (embedded). Therefore, the fillingamount of the magnetic regenerative material 42 is decreased by thevolume of the filler 44A.

The filling rate of the magnetic regenerative material 42 in the centralregion 21 inside the low temperature region 26 b is higher because thecentral hole 45 is opened in the center of the filler 44A correspondingto the central region 21. Meanwhile, the filling rate of the magneticregenerative material 42 in the peripheral region 22 is lower than inthe central region 21 because the filler 44A exists in the peripheralregion 22.

As described, in the regenerative refrigerator of the fourth embodiment,the filling rate of the magnetic regenerative material 42 in the centralregion 21 is greater than the filling rate of the magnetic regenerativematerial 42 in the peripheral region 22 inside the low temperatureregion 26 b. Therefore, inside the low temperature region 26 b, thespecific heat of the central region 21 is larger than the specific heatof the peripheral region 22.

In a manner similar to the regenerative refrigerator 1A of the firstembodiment, the filling amount of the magnetic regenerative material 42can be reduced without reducing a cooling efficiency of the second stageregenerator 26 of the regenerative refrigerator 1D of the fourthembodiment.

FIGS. 7B and 7C illustrate modified examples to the filler 44Aillustrated in FIG. 7A. A filler 44B illustrated in FIG. 7B is formed ofa metallic mesh. The structure of the metallic mesh is not specificallylimited and can be appropriately selected in response to the specificheat and the filling rate of the desirable regenerative material.

A filler 44C illustrated in FIG. 7C is formed so that radiating openings47 extend from the central hole 45 instead of the through holes 46opened in the filler 44A. The radiating opening 47 is shaped like atrapezoid, of which lower base longer than the upper base is connectedwith the central hole 45. By appropriately selecting the shape of theradiating openings 47, the specific heat of the regenerative materialinside the regenerator can be varied. The materials of the fillers 44Band 44C are preferably copper or a copper alloy having a high heatconductivity such as the filler 44A.

Although the outer shapes of the fillers 44B and 44C of the modifiedexample illustrated in FIGS. 7B and 7C are like rings, the outer shapeof the filler is not limited to the shape of a ring. For example, theouter shape of the filler may be a sphere, a cylindrical column, arectangular solid, or the like.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions, nor does theorganization of such examples in the specification relate to a showingof the superiority or inferiority of the invention. Although theregenerative refrigerator has been described in detail, it should beunderstood that various changes, substitutions, and alterations could bemade thereto without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A regenerative refrigerator comprising: aregenerator filled with a regenerative material for accumulating coolingof a refrigerant gas, wherein the regenerator is divided into a centralregion and a peripheral region on a cross-sectional face of theregenerator, and a specific heat of the central region is larger than aspecific heat of the peripheral region.
 2. The regenerative refrigeratoraccording to claim 1, wherein a flow path resistance for the refrigerantgas in the central region is less than a flow path resistance for therefrigerant gas in the peripheral region.
 3. The regenerativerefrigerator according to claim 1, wherein a magnetic regenerativematerial made of a magnetic material is provided in the central region,and a nonmagnetic regenerative material made of a nonmagnetic materialis provided in the peripheral region.
 4. The regenerative refrigeratoraccording to claim 1, wherein the central region and the peripheralregion are divided by a separating member.
 5. The regenerativerefrigerator according to claim 1, wherein a magnetic regenerativematerial made of a magnetic material is provided in both of the centralregion and the peripheral region, and a filler is provided in theperipheral region.
 6. The regenerative refrigerator according to claim5, wherein a filling rate of the magnetic regenerative material islarger in the central region than in the peripheral region.
 7. Theregenerative refrigerator according to claim 5, wherein the filler is ametallic mesh.
 8. The regenerative refrigerator according to claim 5,wherein the filler has a plurality of through holes.